Your account number (or client number) is your unique client number or ID.
You can obtain your client number in a number of ways but the easiest is to log into the admin panel. On the main screen (the one you land on), you will see something along these lines:
In the above screenshot, you can clearly see, on the right-hand side all of the following information:
- the client number (id)
- your username
- your user number (id)